  # CS601 - Data Communication - Lecture Handout 17

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# Conversion

## Digital To Analog Conversion

• Process of changing one of the characteristics of an analog signal based on the info in a digital signal
• When you Tx data from one computer to the other using a public telephone line
• Original data is digital but because telephone wires carry analog signal, original data must be converted
• Digital data must be modulated on an analog signal that has been manipulated to look like two distinct values corresponding to binary 1 to binary 0 • Figure shows the relationship b/w digital info the digital to analog conversion hardware & resultant analog signal

### Variation in Characteristics of Sine Wave

• A sine wave is defined by 3 characteristics:
• Amplitude
• Frequency
• Phase
• By changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data
• When we vary any one of these characteristics ,we create a second version of that wave
• If we than say that the original wave represents binary 1,the variation can represent binary 0 or vice versa
• So by changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data

### Mechanisms for Modulating Digital Data to Analog Signals

• Any of the three characteristics listed above can be altered in this way, giving us at least 3 mechanisms for modulating digital data into analog signals
• Frequency shift keying(FSK)
• Phase shift keying (PSK)

### Fourth Mechanism

• In addition, there is a fourth and better mechanism that combines changes in both amplitude and phase called Quadratue Amplitude Modulation(QAM)
• QAM is the most efficient of these options and is the mechanism used in all modern modems

### Types of digital to analog modulation ### Aspects of Digital to Analog Conversion

• Before we discuss specific methods of digital to analog modulation,two basic issues must be defined:
• Bit/Baud rate
• Carrier signal
• Bit Rate & Baud Rate
• Two terms used frequently in data communication
• Bit rate
• Baud rate
• Bit rate: no of bits transmitted during one second
• Baud rate: no of signal units per second that are required to represent that bit

### Bit Rate & Baud Rate

• In discussion of computer efficiency, bit rate is more important –we want to know how long it takes to process each piece of info
• In data transmission, however ,we are more concerned with how efficiently we can more that data from place to place, whether in pieces or blocks
• The fewer signal units required, the most efficient the system and less bandwidth required to transmit more bits ,so we are more concerned with baud rate
• The baud rate determines the B.W required to send the signal

### Relationship b/w bit rate &band rate

• Bit rate equals the baud rate times the no. of bits represented by each signal units
• The baud rate equals the bit rate divided by the no. of bits represented by each signal shift
• Bit rate is always greater than or equal to Baud rate

### Analogy for Bit rate &Baud rate

• In transportation a band is analogous to a car,a bit is analogous to a passenger
• A car can carry one or more passengers
• If1000 cars can go from one point to another carrying only one passenger(only driver),than 1000 passengers are transported
• However, if each car carries four passengers, then 4000 passengers are transported
• Note that the number of cars, not the numbers of passengers determines the traffic and therefore the need for wider highway
• Similarly, the baud determines the required bandwidth, not the bit rate

### Example 5.6

An analog signal carries 4 bits in each signal element.If 1000 signal elements are sent per second, find the Baud Rate and Bit Rate?

## Solution:

• Baud Rate= Number of Signal Elements
• Baud Rate =1000 bauds/second
• Bit Rate=Baud Rate * Number of bits per signal element
• Bit Rate= 1000 * 4 = 4000 bps

### Carrier Signals

• In analog TX. The sending device produces a high frequency signal, that acts as a basis for the information signal
• This base signal is called the Carrier Signal or Carrier Frequency
• The receiving device is tuned to the frequency of the carrier signal that it expects from the sender
• I=Digital info is then modulated on the carrier signal by modifying one or more of its characteristics (Amplitude, Frequency, Phase)
• This kind of modification is called Modulation and info signal is called a Modulating Signal

• In ASK, the strength of carrier signal is varied to represent binary 1 or 0
• Both frequency and phase remain constant, while the amplitude changes
• Which voltage represents 1 and which represents 0 can be chosen by System Designer
• A bit duration is the period of time that defines one bit
• The peak amplitude of the signal during each bit duration is constant and its value depends on the bit (1 or 0)
• Speed of transmission during ASK is limited by the physical characteristics of Tx. Medium ### Effect Of Noise on ASK

• ASK is highly susceptible to noise interference
• NOISE: Unintentional voltages introduced onto a line by various sources such as Heat or Electromagnetic Radiation from other sources
• These unintentional voltages combine with signal to change the amplitude
• A 1 can be changed to 0 and a 0 to a 1
• ASK relies solely on Amplitude for recognition
• Noise usually affects the amplitude, therefore ASK is the modulating method, that is most affected by Noise

• On-Off Keying (OOK)
• In OOK, one of the bit values is represented by no voltage
• The advantage is the reduction in the amount of energy required to transmit Information
• Bandwidth of a signal is total range of frequencies occupied by that signal
• When we decompose an ASK modulated signal, we get a spectrum of many simple frequencies
• The most significant ones are those b/w, fc-Nbaud/2 and fc+Nbaud /2 with carrier frequency fc at the middle • Bandwidth requirements for ASK are calculated using the formula:
• BW=(1+d) * Nbaud
• BW = Bandwidth
• Nbaud= Baud Rate
• d= factor related to condition of line (min.value = 0)

### Example 5.8

Find minimum bandwidth required for an ASK signal TX at 2000 bps. TX. Mode is half duplex

## Solution

• In ASK, Baud Rate= Bit Rate
• Therefore, Baud Rate = 2000
• Also ASK requires a minimum bandwidth equal to its Baud Rate
• Therefore Minimum BW = 2000 Hz

## Summary

• Digital to Analog Conversion
• Bit Rate and Baud Rate
• Carrier Signals